This invention relates generally to an apparatus for measurement of electrical properties in a semiconductor wafer, and, in particular, the measurement of the electrical properties of a dielectric layer in a semiconductor wafer body without invading the semiconductor wafer material. The determination of the electrical properties of a dielectric layer in a semiconductor wafer body is a critical factor in the production of such wafers as is well known in the art. Measurements such as dielectric rupture voltage, dielectric field strength, time dependent dielectric breakdown and oxide charge measurements, for example, are typically accomplished by first fabricating metal or doped polysilicon mesas over the dielectric layer that serve as electrical contacts to which measurement probes may be applied. The metal mesas or dots, together with the dielectric layer and substrate form an metal-oxide-semiconductor structure. Fabrication of the metal mesas is a time-consuming and costly operation. It typically involves the application of an aluminum metal layer above the dielectric or oxide layer. An organic layer is then applied in a masking operation followed by an acid etching to leave the metal mesas in the desired areas. A sintering and alloying process is then completed to make a chemical bond between the metal mesas and the dielectric or oxide layer.
Also, as is well known in the art, mesa fabrication method may create inaccurate measurements because of the invasiveness of the technique with the wafer structure.
An alternative to mesas is described in an article entitled "Vacuum Operated Mercury Probe for CV Plotting and Profiling" by Albert Lederman, Solid State Technology, August 1981, pps. 123-126. This article discloses utilizing mercury contacts for replacing aluminum in capacitive voltage measurement techniques designed to characterize semiconductor properties. The Lederman paper discloses a vacuum operated mercury probe for doing measurements of MOS, homogeneous, non-homogeneous semiconductor wafers and semiconductor wafers on insulating substrates. Problems may arise utilizing the Lederman mercury probe in that mercury may react chemically with the materials of the wafer under study. Mercury also poses a significant safety problem in its use, and mercury sublimes at elevated temperatures when accelerated temperature testing of the semiconductor wafer is desired. Thus, a mercury probe has limited application.